Which nerves and pathway are involved in the pupillary light reflex?

• A. Afferent limb CN II; efferent limb CN IV; pathway via lateral geniculate nucleus
• B. Afferent limb CN II; efferent limb CN III; pathway via pretectal area to Edinger-Westphal nucleus to the ciliary ganglion and pupillary sphincter
• C. Afferent limb CN I; efferent limb CN III; pathway via hypothalamus
• D. Afferent limb CN II; efferent limb CN III; pathway via superior colliculus

Answer: (B)

The pupillary light reflex is driven by a direct afferent and a parasympathetic efferent arc that link the eye to the midbrain structures controlling the pupil. Light detected by the retina travels via the optic nerve (afferent limb) to the pretectal area in the midbrain. From there, signals are sent bilaterally to Edinger–Westphal nuclei. The Edinger–Westphal nucleus then sends parasympathetic fibers along the oculomotor nerve to the ciliary ganglion, whose short ciliary nerves stimulate the sphincter pupillae to constrict the pupil. This sequence—afferent input from CN II, efferent output via CN III, with the relay through the pretectal area to the Edinger–Westphal nucleus to the ciliary ganglion and pupil sphincter—best fits the described pathway.

Reasons the other ideas don’t fit: the efferent limb would be the oculomotor nerve, not CN IV (which innervates the superior oblique). The pathway through the lateral geniculate nucleus is for conscious visual perception, not the reflex controlling pupil size. The olfactory nerve (CN I) is unrelated to vision or pupil constriction, and the hypothalamus or superior colliculus are not the primary relay points for this reflex arc.